The invention relates to a method for welding a double layer sheet metal, comprised of two cover sheet metals and an intermediate layer of a combustible material, in particular, of impregnated paper, with a bonding sheet metal by resistance or resistance roll seam welding, wherein the first cover sheet metal rests against the bonding sheet metal and one of the oppositely positioned welding electrodes contacts the second cover sheet metal and the other one of the welding electrodes contacts the bonding sheet metal, respectively, for performing the welding process, so that the current between the welding electrodes first flows via the second cover sheet metal, at least one shunt positioned adjacent to the welding location, the first cover sheet metal, and the bonding sheet metal, so that thereby the intermediate layer is heated and removed in the area of the welding location and, subsequently, the cover sheet metals, with the bonding sheet metal, are welded together by directly contacting one another.
Double layer sheet metals are known in various embodiments. In a known method for manufacturing such a double layer sheet metal of the aforementioned kind (Patent Abstracts of Japan, vol. 013, No. 060 (M-796), Feb. 10, 1989, and JP 63 264279 A; and Patent Abstracts of Japan, vol. 10, No. 301 (M-525), Jun. 3, 1986, and JP 61 115687 A), one of the cover sheet metals and the bonding sheet metal are contacted by two electrodes which are of identical size and limited to the area of the welding location. The shunt is formed by an outer bridge which contacts one of the cover sheet metals and the bonding sheet metal far outside of the welding location. In order to prevent that, when a voltage is applied, the current density at the relatively small contact locations of the welding electrodes is too great, the first mentioned prior art proposes to affect the current conduction by controlling the applied voltage such that, first, a current of low current intensity flows which results in melting of the intermediate layer comprised of resin. As soon as the two cover sheet metals, with displacement of the melted intermediate layer, have been contacted as a result of the clamping force acting on the one cover sheet metal and the bonding sheet metal, which can be detected by a change of the applied voltage, the current between the electrodes can flow directly via the cover sheet metals and the bonding sheet metal. From this point in time on, the current is increased to the required welding current intensity and the welding connection between the bonding sheet metal and the cover sheet metals is produced. This method thus requires current and voltage control during the bonding process.
In a known double layer sheet metal (DE 195 03 166 A1), of which at least one cover sheet metal has knobs, the two cover sheet metals are welded to one another at the end faces of the knobs wherein, however, not all knobs must be welded to the other cover sheet metal. Such double layer sheet metals are characterized by a high geometrical moment of inertia while having a relatively small weight per surface area. Because of this property, they are pre-destined to be used in motor vehicles.
Moreover, it is known to fill the hollow space between the two cover sheet metals with a filler. This filler can act as a reinforcing means and as a noise dampening means. The manufacture of such a double layer sheet metal with an intermediate layer is carried out such that the intermediate layer is placed as a perforated mat onto the sheet metal with knobs such that the end faces of the knobs are exposed. Accordingly, after placement of the other cover sheet metal, the two cover sheet metals can be welded together without problems at the directly contacting end faces of the knobs by resistance welding.
In contrast, the welding of such a double layer sheet metal with a bonding sheet metal by resistance welding is a problem because of the insulating intermediate layer. Welding of the bonding sheet metal with the double layer sheet metal in the area of the end faces of the knobs can be carried out without problems, but not on locations where between the two cover sheet metals the insulating intermediate layer is positioned. Welding experiments on double layer sheet metals with cover sheet metals of 0.2 to 0.4 mm thickness by means of conventional welding electrodes with spherical contact surfaces, wherein one of the welding electrodes contacts directly the sheet metal with knobs and the other welding electrode is contacted indirectly via the bonding sheet metal with the sheet metal with knobs, have shown that, in particular, an annular melt through of the sheet metal with knobs that is contacted directly by the welding electrode can occur. A high-strength connection of the bonding sheet metal with both cover sheet metals of the double layer sheet metal can therefore not be achieved.
It is an object of the invention to develop a method which ensures that a bonding sheet metal can be connected by resistance welding with both cover sheet metals of a double layer cover sheet metal with insulating intermediate layer.
This object is solved with a method of the aforementioned kind such that one of the welding electrodes contacts the second cover sheet metal with a surface area that is at least ten times as large as the cross-sectional surface area of the welding location, that the flowing current burns off, without melting it, the intermediate layer in the area of the welding location by controlled heating of the cover sheet metal and that, subsequently, after the welding location has been completely burned free, both cover sheet metals are brought into electrically conducting contact with the bonding sheet metal by a directed local pressure application only at the welding location and are then welded together therewith.
With the method according to the invention a welding connection of the bonding sheet metal with both cover sheet metals is obtained, because, upon placement of the one welding electrode, as a result of the very large contact surface of the one welding electrode, a current density that is too great and thus overheating of the cover sheet metals at the welding location are prevented, without requiring a limitation of the current flowing during the individual phases of the method. The current flowing in the cover sheet metals is distributed onto a large cross-sectional surface area and results initially only in that the insulating intermediate layer, which is disruptive for the welding process, is burned off at least in the direct surroundings of the welding location. Only thereafter a direct contact of the cover sheet metals at the welding location occurs. Then there is no longer the risk of burning through or of an annular eroding by heat at the contact location of the cover sheet metal at the welding electrode. A sufficiently large current conduction for the burning off process is made possible by the shunt. In the case of a sheet metal with knobs, the shunt may be the knobs to be welded to one another. In a double layer sheet metal without such contact-providing knobs, an artificial contact bridge can be produced between the two double layer sheet metals.
There are several possibilities for the purpose of preventing a current density that is too great at the time of positioning the welding electrodes. According to a first alternative, it is suggested that for the large surface area contacting by one of the welding electrodes, a plate-shaped welding electrode or a ring-shaped welding electrode with a central support pin is used. According to a second alternative solution it is suggested that a lost sheet metal piece is used between the welding electrode and the cover sheet metal which is then also connected by welding. Such a lost sheet metal piece has a also a further advantage because it provides reinforcement.
In order to ensure a direct contact of the cover sheet metals after burning off the intermediate layer, according to one embodiment of the invention it is suggested that the other welding electrode is a welding electrode having a spherical contact surface. When pressure loading occurs, the welding electrode with the spherical contact surface presses via the bonding sheet metal the cover sheet metal into the hollow space of the double layer sheet metal and brings it into contact with the other cover sheet metal.